This invention relates to image sensors for converting an optical image into electrical signals and, in particular, to a pixel element in such a sensor.
One common type of image sensor, commonly found in digital and video cameras, includes an array of photoelements, where each photoelement generates a signal approximately proportional to the light impinging upon the photoelement area. As shown in FIG. 1, such a photoelement array 10 outputs its signals, typically pursuant to an addressing operation, to an analog-to-digital converter 12 to produce digital signals. Processing circuitry 14 then performs the required processing of the digital data to, for example, display the image on a screen or store the image in a memory.
FIG. 2 illustrates one photoelement 20 (or pixel element) in the photoelement array 10 and serves to illustrate a common drawback of photoelements. The circuit of FIG. 1 is described in detail in U.S. Pat. No. 6,037,643, assigned to Hewlett-Packard Company and Agilent Technologies. A similar circuit is described in U.S. Pat. No. 5,769,384, also assigned to Hewlett-Packard Company and Agilent Technologies.
Typically photoelements, such as shown in FIG. 2, generate a charge on an integrating capacitor 22 proportional to the light impinging upon the photoelement and the time the shutter is open (i.e., the integration time). The charge is converted to a voltage outside of the photoelement during a reading cycle. The voltage output is then applied to an analog-to-digital converter, as shown in FIG. 1.
In FIG. 2, a bias current is set up by a bias signal PBB controlling a transistor 24. Transistors 26 and 28 form a bias point amplifier for setting the base bias of a phototransistor 30 at a fixed level with respect to its emitter. Transistors 26 and 28 operate as a negative feedback loop, wherein an increased emitter voltage pulls up the gate of transistor 26, which causes transistor 28, connected as a source follower, to lower the emitter voltage. Transistor 28 also provides isolation of the phototransistor 30 emitter from fluctuations at node 32.
In operation, the integrating capacitor 22 is assumed to be initially charged to a reset voltage by coupling the capacitor to the summing node of the transfer amplifier 44 while read transistor 36 is on. A shutter signal is high during the initial charging of capacitor 22 so that the shutter transistor 38 is off and transistor 40 is on. Transistor 40, when on, provides a path for phototransistor 30 to draw current from the power supply.
When the shutter signal goes low, transistor 40 is turned off and transistor 38 is turned on, discharging capacitor 22 through phototransistor 30 at a rate depending on the light impinging on the base of phototransistor 30. At the end of the shutter period (e.g., 20 microseconds), the shutter signal goes high, decoupling phototransistor 30 from capacitor 22. Since the rate of discharge of capacitor 22 during the shutter period is approximately proportional to the light incident upon the phototransistor 30, the charge on capacitor 22 after the shutter is closed now reflects the integral of the light intensity during the time that the shutter was open.
A read signal NRD then goes low to couple capacitor 22 to an output line 34 and to the input of a transfer amplifier 44. Transfer amplifier 44 converts the charge on capacitor 22 to a voltage signal. The transfer amplifier 44 pulls the output line 34 up to Vref (basically a reset level of capacitor 22), resulting in the charge that was removed from capacitor 22 by the light-induced current during the shutter open time being transferred to a transfer capacitor 48. The read signal is now raised to turn off transistor 36.
The output of the transfer amplifier 44 now corresponds to the amount of light that impinged on phototransistor 30 while the shutter was open. This voltage is processed as shown in FIG. 1 for that particular pixel position. The output line 34 may be connected to all pixel elements in a column, where only one row of photoelements is addressed at a time by the NRD line being common to a row of pixels.
One problem with such image sensors that convert a charge on an integrating capacitor internal to the pixel area to a voltage outside the pixel area is that the transfer capacitor 48 and integrating capacitor 22 must be fairly large to prevent the capacitors"" signals from being significantly distorted by stray capacitances that are coupled to the transfer capacitor 48, the integrating capacitor 22, or any of the interconnects between the two when the read transistor 36 is turned on. Further, the additional charge-to-voltage conversion circuitry takes up chip area.
Accordingly, the design of the pixel element is relatively inflexible, and its sensitivity (ability to produce large signals in low light conditions) is limited due to the required size of the transfer capacitor 48. The size of the transfer capacitor 48 has an inverse relationship to both the settling time of the transfer amplifier 44 and the substrate noise coupling into the signal. This means that as the transfer capacitor is made smaller to increase the sensitivity of the photodetector, the settling time and noise get worse.
What is needed is a photoelement that does not suffer from the drawbacks of the prior art.
A photoelement (or pixel element) for an image sensor is described that does not require a charge-to-voltage conversion, but instead outputs a voltage directly related to the intensity of the light impinging on the photoelement. Hence, a relatively large integrating capacitor is not needed. In one embodiment, only parasitic capacitance is used for the integrating function. Additional capacitance may be added to control the gain of the photoelement.
The integrating capacitance is initially charged to a reset voltage. A shutter signal closes a switch that couples the capacitance to a phototransistor or photodiode to discharge the capacitance. The switch is opened after the shutter period so that the remaining charge corresponds to the integral of the light that impinged on the photoelement during the shutter period.
An MOS transistor, connected in a source follower configuration, has its gate connected to the integrating capacitance and its source coupled to an MOS read transistor. The read transistor is also connected to an output pin of the photoelement. When the read transistor is turned on, the voltage at the source of the source follower is applied to the output pin. There is no external charge-to-voltage transfer circuitry used.
The source follower shields the integrating capacitance from any other parasitic capacitances not intended to be part of the integrating capacitance, thus making the output of the photoelement highly accurate with high gain.